Various types of low-friction coatings are known in the art and are used in a wide variety of applications to allow contacting surfaces to rub against one another with reduced friction and/or wear. Low-friction coatings having improved tribological properties are sought for a wide range of articles and applications, from forming and machine tools, to machine components and bearings, to various types of lightweight alloys, just to name a few.
As an example, low-friction coatings could be used to advantage in various types of internal combustion engines, such as gasoline and diesel engines. More specifically, friction between piston rings and cylinder walls in an internal combustion engine contributes significantly to the overall frictional losses, thus inefficiency, of the engine. In addition, wear of the piston ring/cylinder system shortens the useful life of the engine and contributes to a gradual loss of engine efficiency as the piston ring/cylinder system wears over time.
Partly in an effort to address the friction and wear problems of the piston ring/cylinder system, various types of coatings and surface treatments have been developed for piston rings and cylinders. For example, coatings comprising molybdenum and chromium are commonly applied to piston rings to increase their wear resistance. In addition, other types of coatings, commonly copper and tin coatings, are frequently applied to the piston rings to provide for improved break-in and seating performance. Surface treatments and coatings for cylinder walls have also been developed in an effort to increase the wear-resistance and decrease the friction of the piston ring/cylinder system and are often used in combination with coated piston rings.
While such coatings work well and are widely used in internal combustion engines, continuing improvements in internal combustion engine technology, e.g., in terms of specific power output, emissions reductions, and thermodynamic efficiency, place increasing demands on the piston ring/cylinder system. Such increased demands include increased temperature, gas, and dynamic loadings on the piston ring/cylinder system. Of course, there is also a general desire to continue to improve the friction and wear characteristics of the piston ring/cylinder system while at the same time reducing the overall costs of production, including the costs associated with the application of coatings and surface treatments. Consequently, new and improved coatings and materials are constantly being sought that will provide better performance over conventional coatings.